Water Wheel Types

Part of Hydro Generator

Water wheels convert the energy of flowing or falling water into rotary mechanical power. For thousands of years before steam engines, water wheels ground grain, sawed lumber, hammered iron, and powered entire industrial economies. They remain the most accessible path to mechanical and electrical power in a rebuilding scenario.

Choosing the Right Wheel

The correct wheel type depends entirely on your site conditions — specifically, the available head (height difference between water intake and wheel) and flow volume.

Wheel Type	Head Required	Flow Needed	Efficiency	Best For
Overshot	3-10+ meters	Low to moderate	60-90%	High-head sites with limited flow
Breastshot	1.5-4 meters	Moderate	50-70%	Medium-head sites
Undershot	0.5-1.5 meters	High volume	20-40%	Low-head sites with strong current
Poncelet (improved undershot)	0.5-2 meters	High volume	40-60%	Improved undershot design

Measuring Your Site

Before choosing a wheel type, measure two things: (1) Head — the vertical drop available from water intake to wheel discharge point. Use a water level or plumb line to measure accurately. (2) Flow — time how long it takes to fill a container of known volume. Multiply to get liters per second.

The Overshot Wheel

The most efficient water wheel design. Water is delivered to the top of the wheel, fills buckets that descend under gravity, and empties at the bottom.

How It Works

1. A flume (channel) delivers water to the top of the wheel.
2. Water pours into buckets attached around the wheel’s rim.
3. The weight of water in the descending buckets rotates the wheel.
4. As buckets reach the bottom, they tip and empty.
5. The empty buckets rise on the other side to receive more water.

The overshot wheel captures both the potential energy (weight of the water falling) and some kinetic energy (velocity of the incoming water). This is why it achieves 60-90% efficiency — far more than other wheel types.

Construction

Wheel Structure:

1. Build the hub — a strong wooden or cast iron center that mounts on the axle. The hub typically consists of two solid wooden discs bolted together with the axle passing through the center.
2. Install the arms (spokes) — 6-8 heavy timber spokes radiating from the hub to the rim. Each spoke is mortised into the hub and the rim ring.
3. Build the rim — a circular framework at the outer diameter, constructed from curved timber segments or straight segments joined end-to-end into a polygon.
4. Attach buckets — the key element. Buckets are formed by:
   • Sole boards (bottom/back of each bucket) attached radially around the rim
   • Side boards (shrouds) — two solid disc walls on each side of the wheel enclosing all buckets
   • The buckets should retain water for as much of the descent as possible

Parameter	Typical Value
Wheel diameter	3-8 meters (matched to available head)
Wheel width	0.3-1.5 meters (wider = more flow capacity)
Number of buckets	24-48 (more = smoother rotation, less spillage)
Bucket depth	200-400 mm
Rotation speed	3-10 RPM

The Flume:

1. Build a wooden or stone channel from the water source to the wheel.
2. Position the flume to deliver water at or slightly past the top of the wheel (the “twelve o’clock” position is ideal, but slightly past — one o’clock — prevents backflow).
3. Control flow with a sluice gate — a sliding board that adjusts the water opening.

Bucket Design Is Critical

Buckets must retain water during descent and release it cleanly at the bottom. Curved bucket soles (like a reversed J-shape) hold water longer during descent than flat soles. Buckets that dump water too early on the descending side waste energy. Buckets that hold water past the bottom and up the ascending side actually work against the wheel.

Power Calculation

Approximate power output:

Power (watts) = Flow (kg/s) x Head (m) x 9.81 x Efficiency

Example: 20 liters/second flow, 5 meters head, 80% efficiency:

• Power = 20 x 5 x 9.81 x 0.8 = 785 watts (approximately 1 horsepower)

The Breastshot Wheel

Water enters at approximately the center height of the wheel — the “breast” position.

How It Works

1. Water enters through a sluice at mid-wheel height.
2. The incoming water fills bucket-like chambers on the wheel’s lower half.
3. Both the impact force of the water and the weight of water in buckets contribute to turning the wheel.
4. The wheel sits in a closely fitting masonry or wooden trough (the breast) that prevents water from escaping past the wheel without doing work.

Construction

Similar to the overshot wheel, but with key differences:

1. The breast (trough) — build a curved trough that closely follows the wheel’s circumference from the entry point down to the discharge at the bottom. Gap between wheel and trough should be less than 25 mm.
2. Bucket design — buckets face the incoming water flow and are shaped to capture the water stream efficiently.
3. Sluice gate — positioned at mid-wheel height to control water entry.

Parameter	Typical Value
Wheel diameter	2-5 meters
Speed	4-12 RPM
Efficiency	50-70%
Head utilized	1.5-4 meters

The Breast Is Essential

Without a tight-fitting breast trough, water leaks past the wheel and efficiency drops to undershot levels (20-30%). The breast must follow the wheel’s curve precisely with minimal clearance. Line it with smooth planks or stone and maintain it to prevent gap widening from erosion.

The Undershot Wheel

The simplest wheel — placed directly in a flowing stream, turned by the current pushing against flat paddles.

How It Works

1. The wheel sits in the stream with its lower paddles submerged.
2. The flowing water pushes against the paddles, turning the wheel.
3. Only kinetic energy (water velocity) is captured — gravitational potential is not used.

Construction

1. Paddle boards — flat boards attached radially around the wheel rim. Typically 8-16 paddles.
2. Axle and bearings — mounted above water level on supports on both banks.
3. Channel confinement — build walls on each side to concentrate the flow against the paddles.

Parameter	Typical Value
Wheel diameter	1.5-4 meters
Speed	2-6 RPM
Efficiency	20-40%
Head utilized	0.5-1.5 meters

Limitations

The undershot wheel is the least efficient because:

• Water pushes against the paddle but also pushes back on the paddle leaving the water — net force is reduced
• Much of the water flows under and around the paddles without doing work
• Only kinetic energy is captured; the much larger gravitational potential energy is wasted

When Undershot Makes Sense

Despite low efficiency, undershot wheels are the right choice when you have high flow volume but minimal head. A large, slow river with only 0.5 meters of fall can still power a useful wheel if enough water hits the paddles. Simple construction also makes it the fastest wheel to build in an emergency.

The Poncelet Improvement

The Poncelet wheel (1820s) dramatically improved undershot efficiency by using curved blades instead of flat paddles:

1. Curved blades catch the water smoothly, reducing turbulence losses.
2. Water enters near the bottom, flows up and around the curved blade, and exits near the top of the blade’s arc.
3. This captures more of the water’s kinetic energy.
4. Efficiency increases from 20-40% to 40-60% with no change in head requirement.

Axle, Bearings, and Power Takeoff

Axle Construction

Material	Diameter	Notes
Hardwood (oak)	200-400 mm	Traditional, easy to replace, requires frequent greasing
Cast iron	100-200 mm	Longer lasting, heavier
Wrought iron/steel	75-150 mm	Strongest per unit weight

Bearings

The axle bearings must support the full weight of the wheel plus the water load while allowing free rotation:

1. Wooden bearings — a hardwood block (lignum vitae is traditional) with a semicircular groove matching the axle. Grease with tallow.
2. Bronze bearings — cast bronze saddles with the axle profile. Far more durable.
3. Stone bearings — granite blocks with polished grooves. The ancient choice — nearly indestructible.

Power Takeoff

Convert the slow wheel rotation to useful work:

Method	Purpose
Direct coupling	Millstones mounted on the same axle
Gear train	Speed multiplication for saws, lathes, generators
Belt drive	Flexible power transmission to offset machinery
Crank and connecting rod	Convert rotation to reciprocating motion for pumps, hammers

Maintenance

Task	Frequency
Grease bearings	Daily when operating
Inspect buckets/paddles	Weekly — repair or replace cracked boards
Clear debris from intake	Daily — leaves, branches, and sediment accumulate
Check wheel alignment and balance	Monthly
Inspect and repair the breast/trough (breastshot)	Quarterly
Replace worn axle bearings	Annually or as needed

Common Mistakes

1. Building an undershot wheel where an overshot would work — if you have 3+ meters of head, an overshot wheel captures 2-3 times more power from the same water flow. Always use the highest-efficiency wheel your site allows.
2. Wheel too large for the available head — an overshot wheel’s diameter should be slightly less than the available head. Building a 5-meter wheel on a 4-meter head means the flume cannot deliver water to the top.
3. Leaky breast trough — for breastshot wheels, the gap between wheel and trough is everything. Even 50 mm of clearance can halve efficiency.
4. Too few buckets — more buckets means less water spills between them. Use at least 24 buckets for overshot wheels; 36-48 is better.
5. Neglecting bearing lubrication — a seized bearing destroys the axle and potentially the wheel structure. Grease daily with tallow or oil.

Summary

Water Wheel Types — At a Glance

• Overshot wheels (water enters at top) are the most efficient (60-90%) and best for high-head, low-flow sites
• Breastshot wheels (water enters at mid-height) suit medium-head sites — require a tight-fitting breast trough
• Undershot wheels (current pushes flat paddles) are simplest but least efficient (20-40%) — best for high-flow, low-head sites
• Curved Poncelet blades double undershot efficiency to 40-60% with no additional head required
• Wheel diameter should match available head for overshot, with 24-48 buckets for smooth operation
• Power output = Flow x Head x 9.81 x Efficiency — measure your site carefully before building
• Bearings require daily lubrication; wheel components require weekly inspection
• Always build the most efficient wheel type your site conditions allow